1. Field of the Invention
This invention relates to photo-patternable perfluorinated organometallic silane sol-gels and methods to process the sol-gels into optical waveguides with low optical loss at the telecommunication wavelengths.
2. Description of the Related Art
With the rapid growth of integrated optics, passive and active waveguides are in high demand to implement routing, switching or filtering of optical information. Two-dimensional slab waveguides are obtained by depositing a thin film on a substrate, in which the film's refractive index is higher than that of the substrate and the medium above the film. Most waveguides for integrated optics also confine the optical radiation in the transverse direction leading to rectangular cross-sections with dimensions of a few micrometers. Such waveguides exist in various geometries, including the raised and embedded strip, rib or ridge, and strip-loaded waveguides. Dielectric waveguides have been fabricated using various thin film technologies, including evaporation, sputtering, by epitaxial growth techniques, by ion implantation or ion exchange techniques. All of these techniques, in particular ion implantation, are complex and difficult to implement. Ion exchange is performed on glass substrates and is not compatible with standard silicon processing. Evaporation, sputtering, and epitaxial growth techniques can be implemented on silicon substrates but require multi-step photolithography, followed by etching to define the strip or ridge. Etching is often achieved by reactive ion etching, which leads to walls along the strip or ridge that are rough. These rough walls increase the scattering losses of the waveguide.
For most applications the waveguide should have low optical losses, preferably lower than 1 dB/cm and have high thermal stability. For telecommunication applications, the optical signals have a wavelength between 1300 nm and 1600 nm. Therefore, it is important that the material has low optical losses in this spectral region. As demonstrated with the deployment of silica fibers, glass is a material of choice for the fabrication of waveguides. However, the fabrication of glasses generally requires high processing temperatures (T>1000° C.). Such high temperatures are a severe limitation to the integration of waveguides into optoelectronic circuits.
Sol-gel methods allow the fabrication of glasses from precursors using low temperature processing. Furthermore, sol-gel methods can produce compositions that are not possible with conventional methods. Sol-gels are fabricated at room temperature using a hydrolysis-condensation polymerization reaction of suitable monomers. Early sol-gel materials were obtained from precursors with the general formula M(OR1)4 where O is oxygen, R1 is an alkyl chain with general formula CnH2n+1, and M is a metal or semiconductor that can form bonds with organic compounds through oxygen donor ligands. Examples for M include but are not limited to silicon (Si), aluminum (Al), titanium (Ti), or zirconium (Zr). When monomers with general structure M(OR1)4 are used all four coordinating groups can be hydrolyzed and condensed, leading to a purely inorganic network containing only M—O bonds. Such sol-gels are referred to as purely inorganic sol-gels. For instance, if M=Si, the resulting coating is silica. Inorganic sol-gel films are usually limited to thin films because of the formation of cracks during polymerization and condensation in thick films (thickness>1 micrometer). Thicker films can be fabricated using multilayer approaches.
When at least one of the OR1 alkoxy side-group is replaced with an organic group, the sol-gel with general formula R1′—M(OR1)3 is referred to as an organically modified sol-gel (e.g. ORMOSIL when M=Si). In this case the element M forms one bond with carbon directly. R1′ in this case can be any organic molecule. With such precursors, M—C bonds do not undergo hydrolysis and thus reduce the coordination to the number of M—O bonds. Due to reduced coordination of M, such materials are less sensitive to the formation of cracks. Waveguide films with good optical quality can be fabricated from such materials.
An example of an ORMOSIL 10 is shown in FIG. 1. In this case, precursors have the general formula R1′—M(OR1)3, or R1′R1″—M(OR1)2, where R1 are alkyl chains such as CH3 or CH2CH3, and R1′ are alkoxy substituents such as OCH3 or OC2H5 and R1″ are aromatic substituents such as phenyl or styryl. Possible ORMOSILS include materials with general formula R1′R1″—M(OR1)2 with:                R1=—CH3, —C2H5; R1′=—OCH3, —OC2H5; R1″=—C6F5         R1=—CH3, —C2H5; R1′=—OCH3, —OC2H5; R1″=—C6F4 —CF═CF2         R1=—CH3, —C2H5; R1′=—OCH3, —OC2H5; R1″=—C6F4 —CF═CF2         R1=—CH3, —C2H5; R1′=—OCH3, —OC2H5; R1″=—C6F4 —CH═CH2         R1=—CH3, —C2H5; R1′=—C6F5; R1″=—C6F5         R1=—CH3, —C2H5; R1′=—CH═CH2; R1″=—C6F5         R1=—CH3, —C2H5; R1′=—CH2CH═CH2; R1″=—C6F5         
Unfortunately, conventional ORMOSILs have high optical loss at the telecommunication wavelengths due to a high concentration of carbon hydrogen and oxygen hydrogen bonds.
A typical absorption spectrum 20 of an ORMOSIL is shown in FIG. 2. Two main absorption bands can be observed: a band 22 located around 1350-1450 nm that originates from O—H bonds, and a band 24 located around 1700 nm that is caused by C—H bonds. In the O—H band two sub-bands can be distinguished, one spectrally narrow band 26 centered at 1375 nm, and a broader band 28 on the low energy side. The spectrally narrow band is assigned to isolated O—H bonds while the broad low energy shoulder is assigned to the formation of O—H dimers by hydrogen bonding. When the density of O—H bonds is reduced, the formation of dimers is less likely and the broad band absorption is reduced.
Standard waveguide fabrication includes the use of a photoresist that is coated on top of the sol-gel film. After exposure and development of the photoresist, reactive ion etching techniques are used to define waveguides into the deposited sol-gel material. As mentioned above, this process often generates rough walls along the structures, which in turn generates optical scattering losses. The capability to pattern the sol-gel directly using standard photolithographic techniques would reduce the number of steps required to fabricate the waveguide in the sol gel materials. UV patternable sol-gels can be exposed directly and developed and do not require any dry etching. This leads to structures with lower roughness and consequently low loss. UV patterning through UV cross-linking is well known in organic materials and sol-gels (see for instance Buestrish et al., J. of Sol-Gel Science and Technology, Vol. 20, p 181-186, 2001). FIG. 3 shows chemical functional groups that are commonly used for cross-linking upon exposure to UV radiation. They include include epoxy 30, oxetanyl 31, vinyl 32, methacryloxy 33, acryloxy 34, cinnamate 35, or chalcone 36. After cross-linking the new bonds that are formed between organic chains lead to an insoluble three-dimensional network. Groups 30-34 require the addition of a photoinitiator, while cinnamate 35 and chalcone 36 do not.
Examples of methods of making patterned metal oxide films through the sol-gel method can be found in U.S. Pat. No. 5,100,764 to Paulson et al. However, no methods to make low loss waveguides for the telecommunication wavelength are described.
A well established scheme to reduce loss due to C—H bonds is to replace such C—H bonds with C—F bonds. The loss around 1550 nm is due to overtones of the C—H stretching mode. By changing the nature of the elements involved in the bond, the energy of the vibration eigenmodes will be shifted to different wavelengths (energy of C—H bond stretch is 3500 cm−1 and that of C—F bond stretch is 1000 cm−1). This replacement of C—H bonds with C—F bonds to reduce optical loss is referred to as perfluorination. In addition to its heavy mass, fluorine has two great advantages of hydrophobicity and bond stiffness, which further reduce absorption and improve the material's stability. However, the replacement of C—H bonds with C—F bonds is also known to reduce the refractive index of the material. This is a problem because to define a waveguide in a sol-gel, the refractive index of the sol-gel must be higher than that of the substrate.
Organically modified sol-gels generally have a refractive index that is lower than silicon when they contain a high number of C—F bonds. These C—F bonds are required to reduce the absorption at the telecommunication wavelengths. Therefore, a low index sol-gel or other organic buffer layer is required between the sol-gel waveguide and the substrate thereby increasing the number of fabrication steps. The index of the buffer layer is generally reduced by further increasing the degree of fluorination in the sol-gel precursors. Following this approach, the index difference between cladding and the waveguide core is small and limits the optical confinement. This in turn, limits the bending radii of the waveguide structures and consequently the footprint of complex waveguide structures such as arrayed waveguide gratings.
Another way to increase the refractive index of a silica based material is to insert titanium oxide. This can be done for instance by doping the glass with nanodispersion of titanium dioxide as described in U.S. Pat. No. 5,840,111 to Wiederhoft et al. or by inserting metals such as titanium or zirconium into the sol-gel composition itself as described in U.S. Pat. No. 4,814,017 to Yoldas et al. Unfortunately, organoalkoxysilane/metal oxide with general formula R1—[Si(O)3]n—[Ti(O)3]m—R1 are not photopatternable and have high optical loss.
Known sol-gels and methods of fabrication have failed to produce high refractive index sol-gels that can be directly fabricated onto silica-on-silicon substrates or photo-patterned into complex waveguide structures with low bending losses and propagation losses below 1 dB/cm at the telecommunication wavelengths.